1. Field of the Invention
The present invention relates to a transmission system for a hybrid electric vehicle, and more particularly, to a transmission system for a hybrid electric vehicle which implements an electric vehicle (EV) mode, a power split mode, and a parallel mode of lock-up and overdrive (OD) driving.
2. Description of Related Art
Generally, a hybrid electric vehicle is a vehicle which is driven by effectively combining at least two different kinds of power sources.
The hybrid electric vehicle generally uses an engine and a motor/generator, and uses the motor/generator having a relatively better low-speed torque characteristic as a main power source in a low speed and uses the engine having a relatively better high-speed torque characteristic as the main power source in a high speed.
Therefore, since the hybrid electric vehicle stops an operation of the engine using fossil fuel and uses the motor/generator in a low-speed section, the hybrid electric vehicle may have an excellent effect in improvement of fuel efficiency and reduction of exhaust gas.
Further, a transmission system for the hybrid electric vehicle as described above is classified into a single mode type and a multi mode type.
The single mode type may not require connection units, such as a clutch and a brake for transmission control, but may reduce efficiency and reduce fuel efficiency at the time of high-speed driving and require an additional torque multiplication apparatus so as to apply to a large vehicle.
The multi mode type may have increased efficiency at the time of high-speed driving and may be designed to enable torque multiplication, thereby being applied to a medium-large size vehicle.
Recently, the multi mode type is mainly used rather than using the single mode type. Therefore, a research for the multi mode type has been actively conducted.
The transmission system in the multi mode type is configured to include a planetary gear set, a plurality of motors/generators which are used as, a motor, and a generator, a connection unit which controls rotating elements of the planetary gear set, a battery which is used as a power source for the motor/generator, and the like.
The transmission system in the multi mode type has different operation mechanisms depending on a connection configuration of the planetary gear set, the motor/generator, and the connection unit.
Further, since the transmission system in the multi mode type has characteristics of which durability, power transmission efficiency, a size, and the like vary depending on the connection configuration, a research and development for implementing a power transmission mechanism which is more robust, reduces in power loss, and is compact in a field of the transmission system for a hybrid electric vehicle has continued.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.